a Where designated, mapping experiments using a site-specific cleavage method (RARE cleavage; Riethman et al. 1997) have been done to demonstrate colinearity of the half-YAC insert DNA with the cognate telomere. In the absence of RARE cleavage data, the presence of subtelomeric repeats adjacent to terminal (TTAGGG)n sequences in a half-YAC clone is taken as strong evidence for proximity to the telomere; this has been borne out by the RARE cleavage experiments carried out so far. The BAC clones used to mark two telomeres for which no half-YAC coverage exists (5p and 20q) were identified by their subtelomeric repeat-sequence content, the presence of an internal telomere repeat sequence, and their localization to the telomeric end of a distal contig in the Global BAC map. The cosmid used to mark the telomere of 19q contains subtelomeric repeats and an internal telomere repeat sequence, and forms the telomeric terminus of the 19q metric physical map (http://greengenes.llnl.gov/genome/). On the basis of the known sequence organization of other telomeres, only additional subtelomeric repeat sequence is likely to reside distal to the subtelomeric repeat segments contained in these clones, although the possibility of single-copy DNA distal to them cannot be formally excluded at present

b Telomeres with a frequency of >10% large variant alleles in the small populations sampled are considered to have Hi polymorphism in the context of this paper, and those with <10% large variant alleles are considered to have Lo polymorphism. For the telomeres not listed, no molecular data are available with respect to large-scale variations and the available FISH data are inconclusive with respect to potential large-scale variation. The polymorphism frequencies detected by FISH are minimum numbers, as detection depends upon the variable presence/absence of only one specific FISH probe at the telomere. The size(s) of the polymorphisms cannot be determined by FISH, but are assumed to be at least the size of the probe used (on the basis of similar FISH signal intensities at all sites). Data on polymorphic telomeres are from Riethman, unpubl. results; Wilke et al. (1991); Ijdo et al. (1992); Cook et al. (1994); Macina et al. (1994, 1995); Martin-Gallardo et al. (1995); Reston et al. (1995); Monfouilloux et al. (1998); Trask et al. (1998); van Overveld et al. (2000)